The present invention relates to a cooling system for a gas turbine. More specifically, the present invention relates to a recuperative cooling system in which steam used to cool the turbine section is subsequently directed to the combustor for NOx control and power augmentation.
A gas turbine is comprised of a compressor section that produces compressed air that is subsequently heated by burning fuel in a combustion section. The hot gas from the combustion section is directed to a turbine section where the hot gas is used to drive a rotor shaft to produce power. The combustion section is typically comprised of a shell that forms a chamber that receives compressed air from the compressor section. A plurality of cylindrical combustors are disposed in the chamber and receive the compressed air along with the fuel to be burned. A duct is connected to the aft end of each combustor and serves to direct the hot gas from the combustor to the turbine section.
Unfortunately, the combustion process occurring within the combustors results in the generation of nitrogen oxides ("NOx") in the hot gas. Since NOx is considered an atmospheric pollutant, numerous method have been tried to reduce NOx generation. According to one method, steam is introduced into the combustor to rapidly quench the combustion process to a temperature below that which promotes high NOx generation rates. In such instances, steam is injected either directly into the combustor or into the chamber in which the combustors are located. The gas/steam mixture produced as a result of the injection then flows through the turbine. Thus, in addition to reducing NOx, the injection of steam increases the mass flow of the working fluid expanded in the turbine and, therefore, the turbine power output. However, additional fuel must be burned in the combustor to raise the temperature of the steam to the desired temperature for the hot gas entering the turbine, which is typically in excess of 1100.degree. C. (2000.degree. F.) and may be as high as 1425.degree. C. (2600.degree. F.) or higher.
Another problem associated with gas turbines is the cooling of the turbine components. The turbine section of a gas turbine typically employs a plurality of stationary vanes circumferentially arranged in rows. Since such vanes are exposed to the hot gas discharging from the combustion section, cooling of these vanes is of utmost importance. Traditionally, cooling was accomplished by using compressed air bled from the chamber in which the combustors are located as cooling air. The cooling air was directed through a cavity formed in the airfoil portion of the vane, which is essentially hollow. Typically, a number of small cooling air passages are formed inside the vane airfoil that extend from the cavity to the surfaces of the vane, such as the leading and trailing edges or the suction and pressure surfaces. Often, such as in the case of leading edge cooling, the passages direct the cooling air from the cavity so that it flows over the surface of the vane in a thin film, thereby cooling the vane in what is often referred to as "film cooling." In any case, after the cooling air exits the vane passages, it enters and mixes with the hot gas flowing through the turbine section.
Unfortunately, the traditional approach to cooling the turbine vanes has a detrimental impact on the thermal efficiency of the gas turbine. Although the cooling air eventually mixes with the hot gas expanding in the turbine, since it bypasses the combustion process the work recovered from the expansion of the compressed cooling air is much less than that recovered from the expansion of the compressed air heated in the combustors. In fact, as a result of losses due to pressure drop and mechanical efficiency, the work recovered from the cooling air is less than that required to compress the air in the compressor. In addition, discharging the cooling air into the hot gas flow results in aerodynamic losses as the cooling air mixes with the hot gas.
One approach suggested in the past has been to avoid bleeding compressed air by using steam generated from exhaust heat to cool the turbine components. After flowing through the vanes, the steam is typically discharged into the hot gas flowing through the turbine, as in the case where compressed air is used as the cooling fluid. Unfortunately, this approach does not result in the optimum recovery of the heat absorbed by the steam in cooling the turbine components.
Therefore, it would be desirable to provide an apparatus and method for cooling the stationary vanes in a gas turbine that did not require bleeding air from the compressor and that made optimum use of steam for purposes of NOx control and power augmentation.